The ubiquitin proteasome pathway (UPP) is implicated in the lifecycle of multiple viruses. The UPP regulates a wide array of protein function and cellular processes and many viruses are known to manipulate the host cell UPP to enable replication, egress and immune evasion. Many proteasome inhibitors (PSM Inbs) have a negative impact on viral infections in vitro, but the involvement of the UPP in multiple cellular functions, coupled with the lack of potency, specificity or in vivo stability of the first generatin PSM Inbs, discouraged the consideration that PSM Inbs could be developed safely as an antiviral therapeutic. In 2003, the FDA approval of the first PSM Inb, Bortezomib, for the treatment of multiple myeloma, provided proof-of-principle that PSM Inbs can be developed with acceptable toxicology profile and good pharmacokinetics/ bioavailability. This has led to the development of many second generation PSM Inbs with improved potency, selectivity, and bioavailability-several of which are already in Phase I-III trials for oncologic applications. We wll leverage the considerable pharmaceutical industry investment already made in bringing these PSM Inbs into clinical trails to accomplish our primary goal: which is to empirically evaluate and re-purpose these bioavailable PSM Inbs as potential broad-spectrum antivirals for infections caused by NIAID Category A-C pathogens. We have already published that Bortezomib inhibits Nipah virus replication with an IC50 100-fold less than the peak plasma concentrations found in patients. Furthermore, we have also obtained in vitro preliminary data showing that Bortezomib can inhibit the replication of multiple Category A-C pathogens: Filoviridae (Ebola), Paramyxoviridae (Nipah), Bunyaviridae (Rift Valley fever), Flaviviridae (Russian-Spring-Summer encephalitis), and Arenaviridae (Junin). To accomplish our goal, we propose during the R21 phase to (1) evaluate the anti-viral efficacy of selected proteasome inhibitors against a panel of NIAID Category A to C viral pathogens, and (2) elucidate the mechanisms underlying the differential efficacy of the various proteasome inhibitors against distinct viral families. In the 33 phase, we will (3) establish the in vivo efficacy, in small animal models, of the most promising proteasome inhibitors characterized in the R21 phase, and (4) determine the relative barriers to resistance using relevant Category A to C model viruses. The advantages of our strategy are three-fold: (a) focusing on the selected PSM Inbs in clinical development takes advantage of the extensive pharmacokinetic data available, which will help guide animal efficacy studies, (b) targeting a host cell component likely limits the development of resistant and escape mutants, and (c) efficacy data in at least two animal models raises the enticing possibility that some PSM Inbs can be considered for off-label use in the treatment of acute and highly lethal viral diseases with no other treatment options.